[support] Contact us form doesn't work properly

[lamp at afan.net]

Hi guys,
I'm building Drupal based websites for friend's Law firm/NPO and I have
some weird issues with Contact forms.

First, he registered domain kesarandcosolicitors.co.uk. It's Drupal 6 site
(in process to upgrade to Drupal 7). On Contact page, let's say the
recipient email for "Recruitment" category is
recruitment[at]kesarandcosolicitors[dot]co[dot]uk, and for the "General"
category is my lamp[at]afan[dot]net. If I select Recruitment category and
check the "Send yourself the copy" checkbox and submit the form, in
error_log I'll get an error "Error sending e-mail (from lamp at ... to
recruitment at ...)" and the email will not be sent to the recruitment at ...
email. But I will get the copy. And, if I do the same thing for the
General category - everything works fine and I'll get both emails (the
copy and me as a recipient) will arrive.

Mail for kesarandcosolicitors[dot]co[dot]uk are managed on MS Exchange
Mail Server in his office.

Now, I just started building his second website, for his NPO,
kesarandcointervene.co.uk. The Contact form behave the same. No email can
be sent to ****@kesarandcosolicitors.co.uk, but it works just fine for
not-to-domain related email (like Yahoo, Gmail, hotmail...)

Any idea?

Thanks for any help,
LAMP